1. Field of the Invention
This invention relates to a PSK demodulator suitably applied to burst wave PSK signals.
2. Description of the Prior Art
PSK (Phase Shift Keying) is a phase modulation by means of digital signals and one of the simplest PSK systems is two-phase PSK system, by which 0 and 1 in digital signals correspond to 0.degree. and 180.degree. of the carrier phase. In order to reproduce digital signals from received PSK signals, it is sufficient to detect whether the carrier phase is for example 0.degree. or 180.degree.. For this purpose, a carrier phase reference is necessary, with which the received carrier phase is compared in order to know how much the carrier phase is shifted in fact with respect to the original carrier phase.
However, since it is difficult to prepare such a precise phase reference at the receiver side, usually information is sent by using, not absolute phase reference but phase variations. This system is called a DPSK (Differential Phase Shift Keying) modulation system. Demodulation of DPSK signals is effected for example by multiplying received DPSK signals by a carrier reproduced by any means.
FIG. 1 is a scheme showing the demodulation process for such DPSK signals in waveform, in which (A) shows a digital signal indicating information to be sent; (B) represents a modulating signal into which the digital signal (A) is transformed; (C) is a carrier which is to be transformed into a modulated signal; (D) shows a DPSK signal obtained by DPSK modulating the carrier (C) by the modulating signal (B); (E) is a carrier reproduced at the receiver side, which is identical to (C); (F) represents a demodulated information signal obtained by multiplying the DPSK signal (D) by the reproduced carrier (E); (G) indicates a modulated signal obtained by shaping the demodulated information signal (F) in waveform, which correspond to (B); and (H) represents a digital signal obtained by demodulating the modulated signals (G), which represents the original information.
In the process described above the modulating signal (B) is formed, basing on the digital signal (A), according to the following principle.
(i) If a digital signal (A) is 1, the polarity (phase) just before it is inversed. PA1 (ii) If a digital signal (A) is 0, the polarity just before it is maintained. PA1 (i) The phase of the burst wave is inversed in the same way as the DPSK signal (D), corresponding to 1 and 0 in the modulating signal (B). PA1 (ii) The burst wave does not exist at the points where the modulating signal (B) changes as 1.fwdarw.0 or 0.fwdarw.1. Consequently the period of the burst wave is not constant.
In addition the DPSK signal (D) is formed by varying the polarity of the carrier (C), basing on the modulating signal (B) thus obtained, according to the same principle as described above.
In the DPSK modulation mentioned above, demodulation errors are prevented, even if the phase of the reproduced carrier (E) is more or less shifted at the moment of the demodulation, because the system is so constructed that the carrier (C) is phase-modulated.
The digital signal (H) can be obtained by using a simple logic circuit as indicated in FIG. 2, starting from the modulated signal (G) reproduced as described above. In FIG. 2, IN indicates the input terminal; OUT shows the output terminal; 1 is a one-clock delay circuit; 2 is an exclusive OR gate; and 3 is a wave shaping circuit.
Further, the carrier (E) mentioned above can be easily obtained by using a well-known Costas loop circuit as indicated in FIG. 3. In FIG. 3, IN indicates the input terminal; OUT shows the output terminal; 4, 6 and 11 are multipliers; 5 is a 90.degree. phase shifter; 7, 9 and 10 are low pass filters; and 8 is a voltage controlled oscillator (VCO). To the input terminal IN is applied the DPSK signal (D) and from the output terminal OUT is obtained the modulated signal (G). In addition, the carrier (E) mentioned above is obtained from the VCO 8 and further the demodulated information signal (F) is obtained from the multiplier 4.
In this way, it is possible to effect easily the demodulation of the DPSK signal by reproducing the carrier (E) at the receiver side and multiplying the received DPSK signal (D) by it.
Consider now the case where the inputted DPSK signal is burst wave.
FIG. 4 (A), (B) and (D) indicate waveforms corresponding to those shown in FIG. 1 and FIG. 4 (D') is a part of (D) with an enlarged time scale representing a waveform in the case where the inputted DPSK signal (D) is a burst wave.
The characteristic points of the DPSK signal (D') in FIG. 4 are as follows.
It is conceivable to effect the demodulation operation for such a burst DPSK signal (D') just as described above, by using a Costas loop indicated in FIG. 3.
However, since the burst DPSK signal (D') is a burst wave, the signal is 0 in almost all the time intervals. Consequently the control voltage of the VCO 8 is extremely small. As a result, the VCO 8 cannot work effectively and this makes it very difficult to reproduce the carrier.
In consequence, for the burst DPSK signal it is not possible to demodulate it by the carrier reproduction method.